In 1991, one of the co-inventors of the present invention, R. S. Liu, and his co-workers observed that bar shaped (1.5.times.2.times.10 mm) single polycrystalline phase samples of (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 exhibited a high critical current density. Based on magnetic-hyteresis measurements, they reported that (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 showed a critical current density, or J.sub.c of 1.24.times.10.sup.5 A/cm.sup.2, when measured at a temperature of 77K and a magnetic field of 1 T (telsa). This value of critical current density is comparable to that measured from YBa.sub.2 Cu.sub.2 O.sub.7 (1.04.times.10.sup.5 A/cm.sup.2) but considerably greater than the value measured for Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, a Tl-2223 material. This observation has been subsequently reported and/or cited in a number of technical publications. See, e.g., "High critical-current densities in (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 with T.sub.c up to 115K," R. S. Liu, et al., Appl. Phys. Lett., Vol 60, No. 8, February 1992, pp. 1019-1021. See also, "Optimisation of thallium `1223` high-temperature superconducting wire by a single-stage cold deformation process," B. A. Glowacki et al., PHYSICA C, 200, 1992, pp. 140-146; and "Development of Tl-1223 superconducting tapes," D. E. Peterson, et al., PHYSICA C, 199, 1992, pp. 161-170.
Other researchers also reported the high critical current density of Tl-1223 superconducting material. Matsuda, et al, prepared bulk samples consisting of (Tl/Pb)--(Sr/Ba)--Ca--Cu--O with a nominal composition of (1223). The Tl/Pb ratio was 0.5/0.5.about.0.8/0.2, and the Sr/Ba ratio was 0.9/0.1.about.0.6/0.4. Their procedure is summarized below:
Mixing (SrO, BaO, CaO, CuO) Calcination (@870.degree. C., 20 hours) PA1 Mixing (Tl.sub.2 O.sub.3, PbO) Pelletizing (30 mm diameter) PA1 Sintering (800.degree..about.900.degree. C,, 10 hours) Grinding and Pelletizing PA1 Partial Melting (@900.degree..about.1,100.degree. C., 10 .about.120 min.) PA1 Annealing (@850.degree..about.880.degree. C., 10.about.50 hours) PA1 (a) preparing a powder mixture having a nominal composition of (Tl.sub.1-x-y Bi.sub.y Pb.sub.z)(Ba.sub.2-z Sr.sub.z)Ca.sub.2 Cu.sub.3 O.sub.9 ; PA1 (b) placing said powder mixture into a silver tube and drawing and/or swaging said silver tube containing said powder mixture into a wire having a pre-determined diameter, wherein x and y are real numbers between 0.2 and 0.4, and z is a real number between 0 and 2; PA1 (c) rolling said wire into a tape having a pre-determined thickness; PA1 (d) subjecting said tape to a two-stage one-sintering process, said two-stage one-sintering process comprising a first stage at first sintering temperature, a second stage at a second sintering temperature, and a temperature ramp between said first and second temperatures.
They calculated the intra-grain critical current density to be 18,000 A/cm.sup.2 at 77K and 1 T. Using a drawing-rolling method, Matsuda, et al, also fabricated a tape-shaped wire with Au--Pd sheath and the Tl-1223 core prepared above. The critical current density for the TI-1223 tape was measured to be about 500 A/cm.sup.2 at 77K and 1 T.
The high critical current density of the Tl-1223 superconducting material in the presence of a magnetic field (about or greater than 1 T) had been attributed, at least in part, to the pinning centers that were introduced into the bulk material in melt form. By comparing the critical current density of (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 with those of Bi- and Tl-2212 and 2223 [e.g., (Bi or Tl).sub.2 (Sr or Ba).sub.2 CaCu.sub.2 O.sub.8 and (Bi or Tl).sub.2 (Sr or Ba).sub.2 Ca.sub.2 Cu.sub.3 O.sub.10)] compounds, Liu, et al. (ibid) concluded that the high critical density of (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 could also be attributed to the three-dimensional character thereof due to the enhancement of the superconducting coupling along the c-axis between the Cu--O planes resulting in an increase in the critical current density relative to the two-dimensional Bi- and Tl-2212 and 2223 compounds. Kim, et al. proposed an exponential dependence for the Josephson tunneling between the conducting Cu--O layers with the interlayer spacing for various high temperature superconducting materials. In the crystalline structure of Tl-1223 (or Tl1212) compound, the two adjacent CU--O layers are separated by a single Tl--O insulating layer; and the distance between two adjacent CU--O layers is 8.72 .ANG.. This distance is very similar to that in YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO). The small distance between the CU--O conducting planes in Tl1223 and YBCO may have allowed the Josephson interlayer coupling to be short-circuited thus enabling both Tl-1223 and YBCO conducting materials to achieve high critical current densities. However, the Tl-1223 material has an inherent advantage over either Tl-1212 or YBCO, in that the Tl-1223 material has a substantially higher To relative to either Tl-1212 or YBCO. The fabrication of Tl-1223 into practical embodiments, such as conducting tapes, therefore has generated great interest in the applications of superconducting materials.
As mentioned above, Matsuda, et al, used a drawing-rolling method to fabricate a tape-shaped wire with Au--Pd sheath and a Tl-1223 core therein; the critical current density for the Tl-1223 tape was measured to be about 500 A/cm.sup.2 at 77K. Typically, the superconducting tapes are fabricated using a process by which superconducting powder is loaded into a silver tube, which is then subject to a swaging or drawing step, followed by a rolling and/or pressing step, and finally a sintering step. The critical current density for a sintered Tl-1223 tape was measured to be about 6,200 A/cm.sup.2, at 75K. See, "Development of Tl-1223 superconducting tapes," D. E. Peterson, et al., PHYSICA C, 199, 1992, pp. 161-170. Conventionally, the Tl-1223 tape was sintered at a sintering temperature of 800.degree..about.900.degree. C. for 1.about.12 hours, following the rolling or pressing step. To improve its critical current density, the sintered Tl-1223 tape can be subject to a post-sintering pressing process. This typically increases the critical current density of a Tl-1223 tape to about 10,000 A/cm.sup.2 (@77K and 1 .mu.V/cm). Another method to improve the critical current density of a Tl-1223 tape is to employ a double-sintering technique. This technique was disclosed by Y. Torri in Eur. Pat. App. No 91104796.7. In the double-sintering process disclosed by Y. Torri, et al., the Tl-1223 tape, after being pressed or rolled, was sintered at 800.degree. C. for one hour, then pressed and again sintered at 840.degree. C. for four more hours. The critical current density of a Tl-1223 tape after the double-sintering process disclosed by Torri can be increased to above 10,000 A/cm.sup.2 (@77K and 1 .mu.V/cm).
Because of its high critical temperature and inherent critical current density, Tl-1223 superconducting material presents a very promising potential for practical applications. However, the Tl-1223 tapes exhibit disappointingly low critical current density when fabricated using conventional technique. On the other hand, the double-sintering technique, which could more than double the critical current density of the Tl-1223 tapes relative to those fabricated using conventional processes, is too expensive and time-consuming. Therefore, it is highly desirable to develop a new technique which could similarly improve the critical current density of Tl-1223 tapes with minimum modification of fabrication facility and minimum additional cost.